As is well known, every electronic device generates some noise during its operation. This is unavoidable and consequently, there are many reasons why it is desirable to know the noise characteristics of such a device. The most important reason being; it is desirable to know the conditions under which noise can effectively impair the operational utility of the device. Because the environment in which the device is to operate will directly affect the noise level generated by the device, and because this environment may change dramatically during the operational life of the device, some ability to predict the noise performance of the device is desirable. Typically, this prediction is done by determining the noise parameters of the device.
With the noise parameters for a device, the matching conditions which are necessary for the lowest noise operation of that particular electronic device can be determined. This is particularly helpful in the case of transistors. Nevertheless, although microwave transistors are the electronic devices of primary interest for consideration here, the considerations to be given transistors pertain as well to any two-port electronic device. For the specific case concerning microwave transistors, however, there is added capability in that the noise parameters can also be used for transistor modeling. Further, noise parameters allow predictable and producible low-noise amplifier design.
Presently, the extraction of a transistor's noise parameters is made by making noise figure measurements while a tuner changes the reflection coefficient seen by the transistor's input. Unfortunately, the tuners used for this purpose are cumbersome mechanical devices which are complicated to use. Another problem confronted in presently used methods for determining noise parameters is that large values of the reflection coefficient (.GAMMA..sub.s) must be generated. The undesirable result is that this can lead to instability of the electronic device under test. Such instability not only complicates the design of the measurement system, it can lead to the generation of meaningless data. Additionally, the tuners used in these testing procedures are generally very expensive. Comparable electronic tuners have a limited tuning range, require frequent calibration with a microwave network analyzer, contribute their own electronic noise, and have a limit to how high in frequency they can perform.
As is recognized by those skilled in the pertinent art, the basic purpose of any noise parameter representation is to describe the variation in noise figure (F) as a function of a reflection coefficient (.GAMMA..sub.s) presented to the input of the device under test (DUT). The standard formula used in industry is: ##EQU1## in which: F.sub.min =minimum noise figure;
.GAMMA..sub.s 32 source reflection coefficient; PA1 .GAMMA..sub.opt =optimum source reflection coefficient; PA1 R.sub.n =noise resistance; and PA1 Z.sub.0 =characteristic (or normalizing) impedance.
Since Z.sub.0 is constant, and since the object has been to obtain F as a function of .GAMMA..sub.s, the noise parameters F.sub.min, .GAMMA..sub.opt, and R.sub.n are of primary importance.
It happens that with .GAMMA..sub.s =O, the electronic device under test will remain stable. Importantly, as recognized by the present invention, the equivalent noise parameter information may be extracted from the DUT by making a series of noise power measurements under the reflection-less condition wherein .GAMMA..sub.s =0 can be accomplished while making noise power measurements of the device under test. Further, it is known that under this condition i.e. .GAMMA..sub.s 32 0, the noise parameters F.sub.min, .GAMMA..sub.opt, and R.sub.n can be written in terms of an alternate set of noise parameters, namely: 1) the noise power deliverable to the input termination of the device ##EQU2## 2) the noise power deliverable to the output termination of the device ##EQU3## and 3) the noise power correlation between the noise levels at the input termination and the output termination ##EQU4## These alternative noise parameters (referred to as "noise-wave" parameters) are related to the self- and cross-spectral densities of the random noise sources within the DUT. Specifically, as indicated above, conversion between these two parameter sets is possible, and formulas to do so have been given by J. A. Dobrowolski. See J. A. Dobrowolski, "A CAD-oriented method for noise figure computation of two-ports with any internal topology," IEEE Trans. Microwave Theory Tech., vol. MTT-37, pp. 15-20, Jan. 1989.
For the purposes of the present invention, it has been recognized that a combination of noise power level measurements can be used to determine the alternative noise-wave parameters. These noise power level measurements are made with appropriate input signals applied to directional couplers of a six-port passive correlation network.
In light of the above, it is an object of the present invention to provide a noise parameter analyzer which measures the correlation between noise at the input and noise at the output of the device under test with reflection-less loading and which is, therefore, very stable in its operation. Another object of the present invention it to provide a noise parameter analyzer which is capable of high frequency operation. Still another object of the present invention is to provide a noise parameter analyzer which is capable of simultaneously measuring both noise parameters and scattering parameter information. Yet another object of the present invention is to provide a noise analyzer which is robust, easy to use, relatively simple to manufacture, and comparatively cost effective.